Radio-frequency identification (RFID) is an identification technique that involves the use of devices known as RFID tags. An RFID tag is a transponder that can be attached to or otherwise integrated into an item to be tracked. The item may be, for example, a product or pallet of products such as those stored in a distributor's warehouse, or individual products on the shelves of a retailer. The item may also be a person or animal, or simply any item exhibiting a particular state. In any such cases, the RF tag can be used in conjunction with a tag reader to uniquely identify the item, or to specify a condition or state of the item using radio waves. In general, a tag reader emits an interrogation signal, which is received and responded to by the RFID tag, thereby resulting in identification of the tagged item. Other information may also be stored in the RFID tag, such as location, number of items remaining in stock, and condition or state of item (e.g., open or closed state).
There are two main classes of RFID tags: passive and active. A passive RFID tag derives its energy for operation from the RF field emitted by the tag reader, and responds to the reader by modulating the reflectivity of its antenna, thereby returning varying amounts of the incident RF energy. An active or semi-active RFID tag responds to the tag reader using RF energy derived from a battery or other local power source on the tag. An antenna included in the RFID tag radiates energy in the radio frequency spectrum to and from the RFID tag. A benefit of passive configurations over active configurations is that no internal power source is required, so as to allow for lower cost and a smaller tag form factor.
However, there are problems associated with passive configurations. For instance, batteryless RFID tags powered by scavenged RF power exhibit a relatively low power output and low sensitivity, and therefore are ineffective over longer ranges. As such, passive RFID tags are typically limited to applications having a short operation range (e.g., less than a few feet), such as those RFID tags used at point of sale for anti-shoplifting and inventory control. In a more general sense, even RFID tags configured with on-board power sources are range limited to some extent.
What is needed, therefore, are RFID tag designs that exhibit greater range of operation, for both active and passive configurations.